The present invention relates to battery chargers, and in particular to a battery charger suitable for charging the rechargeable battery of cordless power tools.
Cordless power tools have been extensively used in a wide range of applications because of their utility. The cordless power tool employs a rechargeable battery and therefore a battery charger is required for recharging the battery as often as need arises. Since the rechargeable battery rapidly warms up as it becomes fully charged, conventional battery chargers utilize this temperature characteristic as an indication for terminating charging operations. For this purpose, a temperature sensor is included in battery chargers for detecting the temperature of the battery to terminate the charging operation when that temperature exceeds a preset value. In conventional battery chargers, the temperature sensor is connected in a circuit through which a charging current is fed to the battery. Because of this series connection, the temperature sensor tends to be heated by the passing large current especially when the source voltage and/or ambient temperature is relatively high, and as a result, the sensor is operated in response to its own heat to terminate the charging current before the battery is fully charged. In certain types of the conventional battery charger, the temperature sensor comprises a pair of temperature responsive contacts which are disengaged from contact with each other when the environment reaches a preset temperature. This type of sensor has also a disadvantage in that the switching action of the contacts shortens their lifetime due to arcing which would tend to fuse them together resulting in a battery failure. Furthermore, the conventional battery charger tends to reinitiate charging due to a fluctuation in the input voltage causing overcharge if the charger remains connected to the AC power source after it is fully charged; fails to start charging when the source voltage and/or ambient temperature is low; and is incapable of ensuring a constant amount of charge since charging operation cannot be terminated at precise timing.